The invention relates to a magnetic multi-pole arrangement of the nth order to control the trajectories of charged particles.
The focusing of ions or electron beams can be accomplished with electrical or magnetic fields. For focusing by means of magnetic fields, magnetic quadrupole lenses (four-pole lenses) are frequently used. U.S. Pat. No. 4,135,114, for example, discloses quadrupole lens for focusing of the electron beam in a color picture tube. This quadrupole lens consists of a square opening introduced in a plate, whose side edges are magnetized with alternating polarity. In this manner a four-pole magnetic field is formed, whose optical axis z coincides with the direction of propagation of the particle beam. On the x and y axes, the particles in one axial direction are deflected toward the optical axis-focused--and in the other axial direction they are deflected away from the optical axis-defocused.
Magnetic quadrupole lenses are of great importance for the focusing of the particle streams in particle accelerators. In order to achieve a sufficient deflection of the energy-loaded particles, however, strong magnetic fields are necessary. In the case of a quadrupole, for the radial component B.sub.r of the magnetic flux density at a distance r from the optical axis and as a function of the azimuth angle .theta., the following equation applies: EQU B.sub.r =B.sub.T (r/G.sub.o).multidot.sin 2.theta.,
where B.sub.T is the magnetic flux density at the middle of the pole shoe and 2G.sub.0 is the aperture diameter of the opening of the quadrupole that is bounded by the pole shoes. The desired distribution of the magnetic flux density is best achieved, if, as shown in FIG. 1, four pole shoes in the shape of hyperbolas are used, which are electrically excited and have alternating magnetic poles.